Muscarinic acetylcholine receptors are relatively abundant and mediate many of the diverse actions of acetylcholine in the CNS, as well as throughout non-nervous tissues innervated by the parasympathetic nervous system. There are five muscarinic receptor subtypes referred to as M1, M2, M3, M4 and M5 which belong to GPCR superfamily. The odd-numbered muscarinic receptors (M1, M3, M5) couple efficiently through Gq/11, to activate of phospholipase C, that initiates the phosphatidylinositol turnover response. The even-numbered muscarinic receptors (M2, M4) inhibit adenylyl caclase activity via activation of the Gi class of G proteins. M2 and M4 receptors also activate G protein-coupled potassium channel, which leads to hyperpolarization of the plasma membrane of excitable cells.